MSK 011 Transistor Mixer

North Coast Synthesis Ltd. Matthew Skala

This documentation is free: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 3.

This documentation is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR- POSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this documentation. If not, see http://www.gnu.org/licenses/.

2 Contents

General notes 4 Specifications ...... 4 Controls and connections ...... 4 Source package ...... 5 PCBs and physical design ...... 5 Component substitutions ...... 5 PNP transistor modification ...... 5 Modification for ±15V power ...... 6 Use and contact information ...... 6

Safety and other warnings 7

Bill of materials 8

Building the module 9 Preliminaries ...... 9 Some notes on knobs ...... 9 Panel components ...... 10 Fixed resistors ...... 10 Trimmer potentiometer ...... 12 Transistors ...... 12 Diodes ...... 12 Capacitors ...... 13 Power header ...... 13 Final assembly ...... 13

Adjustment and testing 15 Short-circuit test ...... 15 Output oﬀset adjustment ...... 15

Patch ideas 16

Circuit explanation 18 Transistor rules ...... 18 Input buﬀers ...... 18 Mixing network ...... 19 Output ampliﬁer ...... 19 AC coupling network ...... 20

Mechanical drawings 21

3 General notes This manual documents the MSK 011 Transistor sensitive pitch-CV applications if one wants to keep Mixer, which is a utility mixer module for use with the pitches exactly in tune; the North Coast Synthe- both audio and control voltages in a Eurorack mod- sis Ltd. MSK 008 Dual VC Octave Switch, with its ular synthesizer. It is a minimalist design using only precision adders based on op amp chips, may be a six transistors and no integrated circuits. better choice for exact pitch control. The Transis- tor Mixer is intended, instead, for users who want Specifications to embrace the organic variability of classic discrete- This module is designed to mount in 6HP of rack transistor circuits. space in a standard 3U Eurorack case. This module (assuming a correct build using the The module’s maximum current requirement in recommended components) is protected against re- ordinary use is 9mA on the +12V supply and 14mA verse power connection. It will not function with the on the −12V supply. Unusual loads on the outputs, power reversed, but will not suffer or cause any dam- including directly-connected headphones or speak- age. Some other kinds of misconnection may possibly ers and so-called “passive” modules, may cause the be dangerous to the module or the power supply. MSK 011 to draw more than this amount of current. It does not require +5V power. Controls and connections The input impedance is nominally 100kΩ for all Here’s a summary of the items on the front panel of inputs, varying a little with control knob positions. the module. The output impedance is nominally 5kΩ. Separate inputs one for each of the four channels. Note AC and DC coupled outputs are provided. Shorting the top one is normalled to a positive voltage any input or output to any fixed voltage at or between (equivalent to roughly +6V) and the bottom to the power rails should be harmless to the module; a negative voltage (about −6V). patching the MSK 011’s output into the output of gain controls one knob for each channel. These some other module on the same power system should are non-inverting, logarithmic controls. For the be harmless to the MSK 011, though doing that is not top and bottom channels, if there is nothing recommended because it is possible the other module plugged into the corresponding input jack, the may be harmed. knob controls the amount of DC offset. You Voltage gain for each channel can be adjusted be- should set these knobs to zero, that is, fully tween zero and a little more than unity. The mixer counterclockwise, if you are not using them as is designed to handle input and output signals over inputs and don’t want a DC offset. a range of at least ±8V; it is harmless to the module DC output the result of mixing the four inputs. to drive the inputs all the way to the power rails, but AC output same as the DC output, but with a sim- such signals will clip. ple filter to block DC. The cutoff frequency de- The very simple Class A transistor amplifiers used pends on the load you apply, but would typi- in this module are subject to temperature-dependent cally be less than 1Hz, so this output is suitable DC offset. There is a trimmer provided on the circuit even for control voltages if they are reasonably board for overall adjustment, and the DC level can be fast-moving. fine-tuned in many patches with the normalled offset Also note, on the circuit board behind the panel, controls on the front panel; however, in patches where the offset trimmer, labelled “R7 100k offset null.” it is important to avoid any DC offset at all, it may This will have been pre-set when the module was be preferable to use the AC-coupled output. Since it built, but it can be adjusted again later if necessary. also may be difficult to adjust the gain controls to ex- Adjustment range for this trimmer is approximately actly unity, this mixer may not be the best choice for ±2V.

4 Source package con amplifier transistors with gain in the range of a A ZIP archive containing source code for this doc- few hundred; to be honest, they are higher specifi- ument and for the module itself, including things cation than this circuit needs. Any transistors used like machine-readable CAD files, is available from the should be able to handle the full power supply volt- Web site at https://northcoastsynthesis.com/. age (24V) between collector and emitter, and at least Be aware that actually building from source requires about 10mA of current. Low-gain transistors (less some manual steps; Makefiles for GNU Make are pro- than about 30 or 40) may have some impact on the vided, but you may need to manually generate PDFs accuracy and available gain of the mixing, but if you from the CAD files for inclusion in the document, are using such transistors, you probably intend for make Gerbers from the PCB design, manually edit them to have an effect. Any other effects of transis- the .csv bill of materials files if you change the bill of tor substitution are likely to be very subtle. It is not materials, and so on. necessary to match transistors, though if two channel Recommended software for use with the source buffers have very different transistors in them, then code includes: those channels may end up sounding different from • GNU Make; each other. • LATEX for document compilation; When substituting NPN transistors, connect the • LaTeX.mk (Danjean and Legrand, not to be collector, base, and emitter to the pads marked “C,” confused with other similarly-named LATEX- “B,” and “E” on the board according to the pinout automation tools); of the specific transistors you are using, even if that • Circuit_macros (for in-document schematic di- does not match the flat side of the TO-92 outline in agrams); the silkscreen (which refers to the 2N5088 pinout). • Kicad (electronic design automation); Fairchild/ON Semi, formerly the main manufac- • Qcad (2D drafting); and turer of 2N5088 transistors, announced their discon- • Perl (for the BOM-generating script). tinuation while this product was in development. The kicad-symbols/ subdirectory contains my North Coast made a lifetime buy and we should be customised schematic symbol and PCB footprint li- able to continue supplying kits and modules with braries for Kicad. Kicad doesn’t consistently keep 2N5088 transistors for the foreseeable future. As dependencies like symbols inside a project directory, of this writing, Central Semiconductor still makes so on my system, these files actually live in a central 2N5088 transistors. But most other small NPN am- directory shared by many projects. As a result, upon plifier transistors should also work well in this circuit unpacking the ZIP file you may need to do some re- and could be substituted; the transistor selection is configuration of the library paths stored inside the not critical. project files, in order to allow the symbols and foot- On substituting other components: we suggest 1% prints to be found. Also, this directory will probably metal film fixed resistors throughout, because these contain some extra bonus symbols and footprints not days, they are cheap enough to use indiscriminately; actually used by this project, because it’s a copy of however, it would probably still work fine with 5% the directory shared with other projects. resistors in most if not all locations. None of the re- The package is covered by the GNU GPL, version sistors are particularly critical with respect to power 3, a copy of which is included in the file COPYING. handling and we supply different power ratings of resistors for different values according to what we can PCBs and physical design source easily. The output AC-coupling capacitor is This module is built on a single PCB 4.20′′×1.30′′, designed as 4.7µF film for best performance, but it or 106.68mm×33.02mm, which mounts perpendicu- should be safe to substitute one a little smaller, or a lar to the front panel. With about another 1mm of non-polar electrolytic. gap between the PCB and panel, the total depth requirement is 34mm. PNP transistor modification This modification is only appropriate for advanced Component substitutions builders. This circuit should work with almost any NPN tran- Because the circuit is so simple, it is possible to sistors. We ship 2N5088 transistors in our kits and reverse the polarities of the components and build it assembled modules, and these are high-quality sili- with PNP instead of NPN transistors, without any

5 changes to the board layout! This might be useful make further modifications of their own. if one wants to use, for instance, vintage germanium I sell this and other modules, both as fully assem- transistors, which tend to be PNP type. However, bled products and do-it-yourself kits, from my Web it means disobeying the instructions on the board storefront at http://northcoastsynthesis.com/. silkscreen, and attaching the power backwards; so it Your support of my business is what makes it pos- is not for the faint of heart. sible for me to continue releasing module designs for To build using PNP transistors: free. Even if you only use the free plans and cannot • Install PNP transistors on the board according buy the commercial products I sell, any assistance to the silkscreen markings: collectors to “C,” you can offer to increasing the profile of North Coast bases to “B,” and emitters to “E.” Disregard would be much appreciated. For instance, you might the TO-92 outlines on the board, which may or post photos of your completed DIY build on your so- may not match your transistors depending on cial media. The latest version of this document and their pinout. the associated source files can be found at the North • Install the diodes D1 and D2 against the Coast Web site. silkscreen markings, with their cathodes in the Email should be sent to circular pads pointing away from the silkscreen [email protected]. symbol’s stripe. • Install the electrolytic capacitors C2 and C3 against the silkscreen markings, with their positive terminals in the circular pads marked “−.” • Attach the power cable upside down (!), with its striped edge denoting the negative rail at the top of the connector on the module PCB, against the “−12V” marking on the silkscreen.

Modiﬁcation for ±15V power To change this circuit for ±15V power: • Make sure all components (taking particular note of transistors and electrolytic capacitors) are rated for 30V. • Change R17 to 2.4kΩ (was 2.7kΩ). • Change R18 to 6.2kΩ (was 5.9kΩ). • Substitute an appropriate power connection for the Eurorack power header. The resistor changes were determined by simula- tion; I have not built a ±15V prototype for testing.

Use and contact information This module design is released under the GNU GPL, version 3, a copy of which is in the source code package in the file named COPYING. One important consequence of the license is that if you distribute the design to others—for instance, as a built hardware device—then you are obligated to make the source code available to them at no additional charge, including any modifications you may have made to the original design. Source code for a hardware device includes without limitation such things as the machine- readable, human-editable CAD files for the circuit boards and panels. You also are not permitted to limit others’ freedoms to redistribute the design and

6 Safety and other warnings Ask an adult to help you. Some lead-free solder alloys produce joints that North Coast Synthesis Ltd. does not offer war- look “cold” (i.e. defective) even when they are cor- ranties or technical support on anything we did not rectly made. This effect can be especially distressing build and sell. That applies both to modules built to those of us who learned soldering with lead solder by you or others from the kits we sell, and to fully- and then switched to lead-free. Learn the behaviour assembled modules that might be built by others us- of whatever alloy you are using, and then trust your ing our plans. Especially note that because we pub- skills. lish detailed plans and we permit third parties to Water-soluble solder flux must be washed off build and sell modules using our plans subject to the promptly (within less than an hour of application) relevant license terms, it is reasonable to expect that because if left in place it will corrode the metal. Sol- there will be modules on the new and used markets der with water-soluble flux should not be used with closely resembling ours but not built and sold by us. stranded wire because it is nearly impossible to re- We may be able to help in authenticating a module move from between the strands. of unknown provenance; contact us if you have ques- Residue from traditional rosin-based solder flux tions of this nature. can result in undesired leakage currents that may af- For new modules purchased through a reseller, fect high-impedance circuits. This module does not warranty and technical support issues should be use any extremely high impedances, but small leak- taken to the reseller first. Resellers buy modules from age currents could still reduce its accuracy. If your North Coast at a significant discount, allowing them soldering leaves a lot of such residue then it might be to resell the modules at a profit, and part of the way advisable to clean that off. they earn that is by taking responsibility for support- Voltage and current levels in some synthesizer cir- ing their own customers. cuits may be dangerous. We also sell our products to hobbyists who enjoy Do not attempt to make solder flow through the tinkering with and customizing electronic equipment. board and form fillets on both sides of every joint. Modules like ours, even if originally built by us, may Some soldering tutorials claim that that is desirable be quite likely to contain third-party “mods,” added or even mandatory, it does look nicer, and it may or deleted features, or otherwise differ from the stan- happen naturally when the conditions are good and dard specifications of our assembled modules when the leads happen to be small in relation to the holes. new. Be aware of this possibility when you buy a But with large wire leads that just fit in the holes, used module. when the holes are connected to the ground plane Soldering irons are very hot. (even through thermal reliefs), on some harder-to- Solder splashes and cut-off bits of component wet lead finishes, with lead-free solder, and so on, leads can fly a greater distance and are harder to you may only end up dumping excessive heat into clean up than you might expect. Spread out some the joint and damaging the components while you newspapers or similar to catch them, and wear eye fuss over perfect fillets. A well-made solder joint that protection. just covers the pad and makes good contact to the Lead solder is toxic, as are some fluxes used with lead on one side of the board, is good enough. lead-free solder. Do not eat, drink, smoke, pick your Building your own electronic equipment is seldom nose, or engage in sexual activity while using solder, cheaper than buying equivalent commercial products, and wash your hands when you are done using it. due to commercial economies of scale from which you Solder flux fumes are toxic, especially from lead- as small-scale home builder cannot benefit. If you free solder because of its higher working temperature. think getting into DIY construction is a way to save Use appropriate ventilation. money, you will probably be disappointed.

7 Bill of materials This table is not a substitute for the text instructions.

Qty Ref Value/Part No. 2 C4, C5 0.1µF axial ceramic 1 C1 4.7µF film, 0.2′′ lead spacing 2 C2, C3 10µF radial aluminum electrolytic, 0.1′′ lead spacing 2 D1, D2 1N5818 or SB130; Schottky rectifier 2 H1, H2 M3x6 M3 machine screw, 6mm body length 2 H3, H4 nylon washer for M3 machine screw 1 J7 male Eurorack power header, 2 × 5 pins at 0.1′′ 6 J1–J6 MJ–3536 switched mono 3.5mm right angle jack, CUI 6 Q1–Q6 2N5088 NPN general purpose amplifier, TO-92 EBC 2 R19, R23 510Ω 1 R17 2.7kΩ 1 R22 4.7kΩ 1 R18 5.9kΩ 1 R21 12kΩ 5 R8–R11, R20 22kΩ 4 R12–R15 47kΩ 4 R2–R5 100kΩ horizontal conductive plastic panel pot, BI Tech- nologies P260P series, audio taper 1 R7 100kΩ horizontal single turn, Vishay T73YP or similar 2 R1, R6 100kΩ 2 R16, R24 330kΩ

Fixed resistors should be 1% metal ﬁlm throughout. Capacitor values are not critical. Transistors may be substituted (see notes). RoHS-certiﬁed zinc-plated steel hardware is recommended, not stainless steel because of galvanic-corrosion incompatibility with aluminum parts. Also needed: solder and related supplies, PCB, panel, knobs, Eurorack power cable, etc.

8 Building the module The circuit board has components on both sides, setscrews in any kits with knobs from batches where which makes the order of assembly important; in- a signficant number of knobs failed testing. stalling the wrong components first may make it dif- Here are some things to be aware of as a kit ficult to safely maneuver the soldering iron to in- builder. stall later components without damaging the already- • Some photos in these instructions were taken installed components. with the older grey knobs, and some dealers may still have kits containing grey knobs in Preliminaries their stock, but newer kits will have blue knobs. Count out the right number of everything according • Do not overtighten the setscrews when attach- to the bill of materials on page 8. ing the knobs! The screw should be tight enough to hold the knob onto the shaft, but there’s no advantage to making it tighter than that, and overtightening may risk destroying the screw thread or damaging the drive slot. • If, despite my efforts to make sure no bad screws get sent to customers, you still get a bad screw that cannot be tightened and no spare for it, then please contact me. • If you want to source an exact replacement for the setscrew, it should be an M3×3mm flat-tip slotted setscrew, which is also sometimes called a “grub screw,” made of RoHS-compliant brass (possibly by exemption). Stainless steel is fine too, and I may sometimes ship stainless steel screws instead of brass if I can find a reliable Some notes on knobs source for them; plain steel should not be used The first batch of knobs I ordered for North Coast here for galvanic corrosion reasons. Hex-socket products turned out to have serious quality problems, screws are fine if you have the driver for them, specifically with the setscrews that hold the knobs but I don’t ship those because I’m not sure all onto the potentiometer shafts. Some of the screws DIY builders do have the right driver. had marginal threads that would strip when the screw • Because it’s a standard M3 thread, in a pinch was tightened, and I ended up having to do a bunch of it’s possible to substitute a plain M3 machine extra testing and ship extra knobs to some customers screw such as are commonly used with Eurorack to replace any that might fail. Later batches have cases, although one of those would obviously also had issues, although they’re under better control look less nice. now because the bad first batch served as a warning to step up the testing procedures. Starting with kits prepared in August 2019, I switched to blue knobs with 100% testing; in September 2020, I switched to a new manufacturer, and knobs that are a slightly darker shade of blue. Although all the knobs I ship in kits now have been tested and passed at least twice, and should be fine to use, I am also shipping spare

9 Panel components The components that go through the panel should be installed first because soldering them may be difficult later. First, place the six jack sockets J1 to J6 in their locations on the board. Note that the silkscreen for these may not be exactly as shown. I had one batch of boards in which the silkscreen lines perpendicular to the panel were missing. The text, and the short lines showing the backs of the jack bodies, should still make clear where the jacks go. At this stage you have the option to leave the panel attached through the rest of the assembly, which will protect the pots from getting bent out of position, or remove it, which will allow for more con- venient access to solder other components. For the photos in these instructions, I have removed the panel for a clearer view of the components, but it may really be a better idea to leave it in place. Without the panel, the pots are supported delicately on their Attach the panel to the jack sockets using the legs and may easily be bent or damaged by careless knurled nuts that came with the jacks. It should be handling. snug, but not tight; you will be removing the panel Note it is not recommended to try to do the jack again soon. sockets and potentiometers in a single step without removing the panel in between, because of the diffi- culty of making the large solder joints on the jacks while the potentiometers are in place without damaging them. This assembly routine, although convo- luted, really seems to be the safest and easiest way to do it. Having parts on both sides of a single board is part of the price paid for keeping the module to only 6HP width.

Solder the jack sockets in place. Fixed resistors Remove the panel, place the four 100kΩ panel po- Resistors are never polarized. I like to install mine in tentiometers R2–R5 in their locations on the oppo- a consistent direction for cosmetic reasons, but this site side of the board, then reinstall the panel. As is electrically unnecessary. In this module, metal film with the jack sockets, it is possible that the silkscreen 1% resistors are recommended for all fixed-value re- markings for these on the board may not be exactly sistors. These will usually have blue bodies and four as shown in the photo here. Use just a nut (not the colour bands designating the value, plus a fifth band other mounting hardware) to hold the potentiometers for the tolerance, brown in the case of 1%. These are in place, and again tighten them only moderately, be- the resistors normally shipped in the North Coast cause this is a temporary assembly. Depending on the kits, but we may occasionally ship better-tolerance board tolerances, the legs of the potentiometers may resistors (such as 0.5%) if we are able to source them be bent somewhat when installed this way. Be careful at a good price. Accordingly, I mention only the four when soldering them not to damage the nearby plas- value band colours for this type of resistor; if you are tic of the jack sockets. Because cleaning these joints using resistors with other codes, you are responsible may be difficult, it is advisable to use no-clean and for knowing them. Note that colour codes on metal not water-washable solder flux. Similarly, because of film 1% resistors are often ambiguous (reading from the limited access it is probably better not to attempt one end or the other end may give two different val- to clip the leads after soldering. ues, both plausible) and some of the colours are hard to distinguish anyway. If in doubt, always measure with an ohmmeter before soldering.

10 Install the two 510Ω (green-brown-black-black) Install the 12kΩ (brown-red-black-red) resistor resistors R19 and R23. These are emitter degener- R21. This resistor is part of the network that links ation resistors for the transistors Q5 and Q6; they the two stages of the output ampliﬁer. make the gain of those amplifying transisters smaller and more controllable.

Install the four 22kΩ (red-red-black-red) resistors R8 to R11 and R20. Most of these provide voltage Install the 2.7kΩ (red-violet-black-brown) resistor pull-down for the input buffers; R22 is part of the R17. This resistor scales the output of the passive network linking the stages of the output amplifier. mixing network to the appropriate range for the output amplifier.

Install the four 47kΩ (yellow-violet-black-red) resistors R12 to R15. These resistors form the mixing network between the input buffers and output ampli- Install the 4.7kΩ (yellow-violet-black-brown) re- fier. sistor R22. This resistor sets the gain for the second stage of the output amplifier. Do not confuse it with the 47kΩ resistors, which have a similar colour code.

Install the two 100kΩ (brown-black-black-orange) resistors R1 and R6. These provide normalling voltages for the top and bottom knobs, to allow intro- Install the 5.9kΩ (green-white-black-brown) resis- duction of a DC offset in control voltage mixing. tor R18. This resistor sets the gain for the first stage of the output amplifier.

11 Install the two 330kΩ (orange-orange-black- “base,” and “collector.” If you are using some other orange) resistors R16 and R24. R16 controls the scale transistors, connect them to match those markings of the offset null trimmer, and R24 eliminates DC off- according to the pinout of the parts you are using, set at the AC-coupled output. even if that puts the flat side of the package other than where it is shown on the silkscreen. The solder pads for these components are smaller and closer together than for any other through- hole components in the project, and the components themselves tend to be relatively heat-sensitive. Solder them carefully, avoiding creating any solder bridges between adjacent pads. Do not use excessive time and heat trying to get the solder to flow through Trimmer potentiometer the board and fillet on both sides, especially not on Trimmers are not exactly polarized, but the three legs pads connected to the ground plane; two-sided fillets of a trimmer serve different functions and need to be may happen naturally, but it is enough for solder to connected to the right holes. The physical arrange- completely cover the pad on one side. ment of the legs and corresponding holes should make Install the six 2N5088 transistors Q1 to Q6. it impossible to install the trimmer wrong way round. Also note that the silkscreened footprint for the trimmer on this board provides a generous amount of space, for maximum flexibility in parts substitution; the trimmers usually shipped in North Coast kits will not fill the space. Install the 100kΩ trimmer R7. This is for adjust- ing the DC offset. Diodes Install the two Schottky diodes D1 and D2. These protect the module against reverse connection of the power supply. They are polarized and must be installed in the correct direction; otherwise they will prevent the module from operating. One end of each diode will be marked, usually with a stripe of grey paint around the black plastic body of the diode. Transistors That end is called the cathode. The diode outline The six NPN transistors in this project are polar- on the PCB silkscreen is marked with a similar stripe ized and must be installed in the correct orientation showing the direction of the cathode, and the solder to work; that orientation is shown by the silkscreen pad for the cathode is square instead of round. symbols. Install each component so that its flat side points in the same direction as the flat side shown on the silkscreen. The three legs of the component must be carefully bent into the same triangular pat- tern (left and right forward, middle backward) as the holes on the board, and then the component pressed into place. There should be a gap of about three millimetres between the board and the component body; do not attempt to seat the component flush on the board because of the risk of breaking off the legs where they enter the body. To aid in modifications with different components, the three holes for each transistor are also labelled with the letters “E,” “B,” and “C,” for “emitter,”

12 Capacitors Power header Install the two 0.1µF ceramic capacitors C4 and C5. Install the 10-pin dual-row Eurorack power header These filter out high-frequency interference on the J7. This header goes on the back of the board, with power supply lines. These capacitors are not polar- C1 and the panel pots, opposite from most of the ized and may be installed in either orientation. other components. It is not polarized in the horizontal plane. However, if it has shorter legs on one side, then those are the ones that should go through the PCB (leaving the longer legs sticking up to mate with the connector on the power cable), and if it has tin plating on one end of the pins and gold on the other, then the tin side should be the one soldered through the board. Secure the header carefully to the board, Install the two 10µF electrolytic capacitors C2 possibly with tape, before soldering it. It is easy to and C3. These filter lower-frequency interference on accidentally solder it at an angle, which is a difficult the power lines. They are polarized components, and error to fix and may cause trouble when you later may explode if connected backwards. As such, there attach the power cable. are multiple clues to help you install them in the right direction. The negative leg of each capacitor will be marked in some way, usually with a printed stripe and minus signs on the plastic wrapping of the capacitor body. The negative leg of the capacitor will usually also be shorter, though that is less reliable than the body markings. On the PCB, the positive and negative pads are marked with positive and negative signs Note that Eurorack power connections are polar- in the silkscreen, and the solder pads themselves are ized even if the connectors are not. The cables are round for negative and square for positive. usually grey ribbon type with a red stripe along one side indicating pin 1, which carries −12V power. For most modules including the MSK 011, the red stripe should be at the bottom when the module is mounted vertically in a case. On the MSK 011, the correct location of the −12V supply is also marked with the text “−12V” and arrows on both sides of the PCB silkscreen. This module is also protected (by the Schottky diodes you just installed) from damage in Install the 4.7µF film capacitor C1. This capaci- case of a reversed power connection; if you connect tor provides AC coupling for the corresponding out- the power backwards and nothing else is wrong, then put. It goes on the back of the board, and needs to be the module will not power up but will be fine once you installed at this point, late in the build, so as not to connect the power correctly. However, many other conflict with the solder connections of other nearby modules are not so protected, and it is dangerous to components. It is not polarized and may be installed get into the habit of depending on protection diodes. in either direction. Destroying a module by connecting power backwards is almost a rite of passage for Eurorack users.

Final assembly If you have removed the panel, reattach it; if you have left it in place, undo the nuts on the potentiometers and add the rest of the hardware. The sequence of hardware for the potentiometers is ﬁrst (nearest the panel) the conical spring washer, high side in the middle and low side around the outside; then the toothed lock-washer; then the nut. In the case of the jack

13 sockets, the knurled nuts provided for these will have screwdriver slots on one side, and those should face the outside with the smoother side facing the panel. Do not overtighten any of this hardware, and be careful, if you are using wrenches or pliers, to avoid scratching the panel. Wrapping the tool jaws with tape may help. Attach the knobs to the potentiometers. Twist each shaft to its limits in each direction to ascer- tain how the slot in the shaft corresponds to where you want the knob pointer, then slide the knob onto the shaft in the correct orientation and tighten the setscrew with a small ﬂat screwdriver. There is a rectangular white area on the back of the board reserved for adding a serial number, sig- nature, quality control marking, or similar. Use a ﬁne-tipped permanent marker to write whatever you want there. Your module is complete.

14 Adjustment and testing The MSK 011 is designed to work with very little makes it easier to avoid mistakes, because the module adjustment. As a result of its minimalist transistor- itself will short together all wires that carry equal based design, there is unavoidably some temperature- potential, making it easier to be sure of testing the sensitive offset in the DC-coupled output. A trimmer relevant adjacent-wire pairs in the cable. is provided to help null this out, but because of the Plug the module into a Eurorack power supply temperature sensitivity the offset will probably never and make sure neither it nor the power supply emits remain at zero in practical use. For audio applica- smoke, overheats, makes any unusual noises, or smells tions it is preferable to use the AC-coupled output. bad. If any of those things happen, turn off the power This adjustment procedure requires the finished immediately, and troubleshoot the problem before module, a smallish cross-tip screwdriver, a suitable proceeding. power supply, and a multimeter. Optional: Plug the module into a Eurorack power supply backwards, see that nothing bad happens, Short-circuit test and congratulate yourself on having assembled the With no power applied to the module, check for short reverse-connection protective circuit properly. Re- circuits between the three power connections on the connect it right way round before proceeding to the Eurorack power connector. The two pins at the bot- next step. tom, marked with an arrow on the circuit board, are for -12V. The two at the other end are for +12V; and Output offset adjustment the remaining six pins in the middle are all ground Turn all knobs fully counterclockwise. Apply power pins. Check between each pairing of these three volt- to the module with nothing plugged into the input, ages, in both directions (six tests in all). Ideally, and measure the DC voltage of the DC-coupled out- you should use a multimeter’s “diode test” range for put. Adjust R7 at the top of the board, labelled this; if yours has no such range, use a low resistance- “offset null,” to bring the output as close to 0V as measuring setting. It should read infinite in the re- is reasonably possible. It is unlikely that you will be verse direction (positive lead to −12V and negative able to get it to stay at exactly zero. lead to each of the other two, as well as positive lead to ground and negative to +12V) and greater than 1V or 1kΩ in the forward direction (reverse those three tests). If any of these six measurements is less than 1kΩ or 1V, then something is wrong with the build, most likely a blob of solder shorting between two connections, and you should troubleshoot that before applying power. Optional: Although we test all cables before we sell them, bad cables have been known to exist, so it might be worth plugging the Eurorack power cable into the module and repeating these continuity tests across the cable’s corresponding contacts (using bits of narrow-guage wire to get into the contacts on the cable if necessary, or probing the pins of the power connector on the back side of the circuit board) to make sure there are no shorts in the cable crimping. Doing this test with the cable connected to the module

15 Patch ideas The basic operation of the MSK 011 is very simple: side, for even-harmonic distortion. it mixes signals. Here, an oscillator and noise source are combined before being fed into a filter. Using the AC-coupled output is appropriate for audio signals where no DC offset is desirable. Remember to turn down the top and bottom knobs, which function as offset generators, if they are not in use.

Each channel has maximum gain somewhat over unity, but to boost a signal further, you can feed it into two or more channels and turn them all up. Shown here with a passive cable-adapter module; you might use a patch like this to boost an external signal a little bit. This patch combines three slow sine waves from an MSK 010 to generate a melody. The top knob is left unpatched and can be turned up to generate a positive oﬀset, shifting these bipolar signals into the 0...5V range needed by the quantizer. Note use of the DC-coupled output to preserve the voltage levels.

For more gain, use feedback from the DC-coupled output to a channel and then take the result from the AC-coupled output. This arrangement will amplify any DC offset too, so it may require some careful adjustment of the top or bottom knobs to cancel out that effect. Voltage gains of up to 10 seem to be rea- Applying a DC offset (top and bottom knobs), sonably stable; in principle there is no limit. Feed- then removing it with the AC-coupled output, allows back through the AC-coupled output would cancel the module to clip signals in a controlled way on one most of the offset, but may have phase problems. The mixer will probably not oscillate under any purely self-patched configuration, though (like any mixer) it could be combined with other modules, such as fil-

16 ters, into more complicated feedback patches.

17 Circuit explanation The MSK 011 is an attempt at minimalist design; the Input buffers simplest transistor-based mixer circuit that will still The MSK 011 contains four input buﬀers. Here is the be usable. It is designed with modern components, schematic for one of them; the others are identical. which makes some issues easier to address, but the +12V basic operating principles go all the way back to the earliest transistor (and, to some extent, vacuum tube) R2 amplifying circuits. Q1 100kΩ Transistor rules Consider a simple NPN bipolar transistor. C R8 B 22kΩ

E −12V The behaviour of all the transistors in the MSK 011 can be understood by two simple rules. The signal enters from the right and is attenuated These are only approximations, and they are specific by the potentiometer R2. Depending on the position to NPN transistors operating in the mode used here. of the knob, somewhere from 0 to 100% of the input Current rule: The current flowing into the collec- voltage is applied to the base of Q1. Then, by the tor (C) is approximately equal to the current flowing voltage rule, the transistor will drive its emitter to out of the emitter (E), and there is approximately no 0.7V less than the voltage applied to the base. That current through the base (B). will draw some amount of current through the resistor Voltage rule: The base is approximately 0.7V R8, and the same amount of current is drawn from more positive than the emitter, and the collector is the power supply at the transistor collector. whatever voltage it needs to be to obey the current The load at the input jack looks like a fairly con- rule, provided that is above the emitter voltage. stant 100kΩ resistance; the transistor only places neg- These rules cut in all directions. A transistor will, ligible load on whatever is driving the input. Mean- to the extent it can, change the voltages and currents while, we can push or pull a relatively large amount at all of its terminals in order to obey the rules— of current through the output and the transistor will changing the voltage at the emitter to be 0.7V less adjust its current draw from the positive power sup- than the base when the base is driven to a specific ply to keep the output at the right voltage, always voltage by external circuity, but also changing the 0.7V less than the attenuated input. If something voltage at the base to 0.7V above the emitter when else tries to pull the output up, then up to the limit of the emitter is driven to a specific voltage, and so on. the roughly 500µA of current that is flowing through If the transistor cannot obey the rules, then they are R8, the transistor will just provide a smaller share of not sufficient to understand the circuit in question that current and allow the external circuit to provide and we need to apply other, more detailed, theory. the rest, keeping the voltage across the resistor un- But in the MSK 011 during normal operation, these changed. Similarly, if the external circuit tries to pull simple rules are a good description of what is go- the output down, the transistor (up to its limits) will ing on. just draw more current from the positive supply to satisfy the external circuit while keeping its emitter 0.7V below its base.

18 Therefore, what happens at the output of this cir- R12 47kΩ cuit in terms of current draw is not seen at the input. The input is said to be “buffered” against fluctuations R13 47kΩ in the output. Such a buffer is useful in this mixer module because it helps prevent crosstalk between R14 47kΩ the input channels. This kind of transistor circuit is called an emitter follower, because the voltage at the emitter follows R15 47kΩ or copies (except for the 0.7V offset) the voltage applied to the base. It is also called a common collector amplifier; the collector of the transistor is connected R16 R17 to the “common” connection of the positive power 330kΩ 2.7kΩ supply. Although this circuit has no voltage gain (it adds an offset, but does not multiply voltage changes in the input by anything other than 1), it serves as a −12V current amplifier because the current available at the +12V −12V output is potentially much more than that consumed R7 100kΩ at the input. One detail not shown on the above diagram, but Note the low-valued resistor R17, just 2.7kΩ to visible in the full schematic on page 22, is the in- the negative supply. The other resistances are much put normalling: channels 1 and 4 have the switching larger, making this network function as a fairly strong contacts on their jack sockets connected to the power fixed attenuator; the large signal range at the input supplies through 100kΩ resistors, so that if there is no translates into a small range at the input of the next signal plugged into these channels, you can turn up stage. their knobs to create a DC offset (positive for channel 1, negative for channel 4). Because of the 100kΩ Output amplifier resistors in series with the 100kΩ resistance of the at- The signal from the mixing network needs to be am- tenuator pot, the maximum offset in either direction plified back up to modular level, and shifted to be is about 6V. centred on zero. +12V Mixing network After buffering, the four input signals go through a +12V passive mixing network which combines them in equal R22 proportion, attenuating them and shifting them into 4.7kΩ a range of voltages near the negative power supply, R18 in order to hit the desired input range for the output amplifier. This network is adjustable with a trimmer 5.9kΩ pot on the circuit board, to compensate for varia- R20 22kΩ tions in the offsets of the transistors throughout the Q6 circuit, which are only (under the voltage rule above) approximately 0.7V each. R21 Q5 12kΩ R23 −12V 510Ω R19 510Ω −12V

−12V

19 This is a two-stage common emitter amplifier; at a lower gain, which makes it easier to control their the emitter of each transistor is connected (through gains precisely and reduces risks from things like feed- the low-value resistors R19 and R23) to the common back. The 2N5088 transistors recommended here can −12V power supply. run comfortably at much higher gain per stage, but As the input signal runs over its small range near using them that way would make the response more the lower power supply voltage, the transistor Q5 dependent on variation among individual transistors, (under the voltage rule) forces its emitter to follow temperature, and so on. that voltage, approximately 0.7V down. This voltage across the 510Ω resistor R19 induces a proportional AC coupling network current. Then, under the current rule, the transis- The output from Q6 is used directly as the DC- tor draws the same amount of current through the coupled output of the module. Having a DC-coupled 5.9kΩ resistor R18, which causes the voltage across output is more or less necessary in a Eurorack mixer that resistor to also vary in proportion. The collector because people want to use them for DC control volt- voltage of Q5 floats to take up the slack voltage, in ages. However, it is an unavoidable fact of using such accordance with the transistor voltage rule. But be- a minimal circuit design that there is some depen- cause R18 is a bigger resistor, the same current varia- dence on transistor behaviour, and in particular, on tion causes a larger voltage variation across R18. The temperature. That offset between base and emitter circuit exhibits voltage gain: a small voltage change that I called “approximately 0.7V” actually depends on the input results in a larger voltage change on the a lot on temperature, and a little on the individual output. transistors. One consequence is that the exact ranges As a first approximation we can say that the volt- of signal voltages will shift with temperature; zero age gain of the first stage (Q5) is equal to R18 di- voltage on all inputs to the mixer may not give zero vided by R19; a change of 1V on the transistor base volts at the DC-coupled output. The offset trimmer and therefore on R19 results in 1/R19 current change can be adjusted to eliminate the offset at some typ- which results in R18/R19 voltage change on R19. Re- ical temperature, but as the temperature changes, it ally, the gain is a little less than that because of the will fall out of trim again. current consumed by the resistive coupling network So for use with audio signals, the MSK 011 also between Q5 and Q6. A more accurate estimate comes provides an AC-coupled output, using a simple RC from measuring the ratio between R19 and the par- high-pass network. allel combination of R18 with the R20+R21 network, C1 4.7µF which gives a voltage gain of −9.86. The voltage gain is negative. Lower voltages on the input give higher voltages on the output (at the collector of Q5), be- R24 cause lower input voltages mean less current through 330kΩ R19, less current through R18, and therefore less voltage drop across R18, bringing the collector of Q5 and low end of R18 closer to the positive supply. The inverted signal at the collector of Q5 goes Applying the formula for RC cutoff frequency through the resistor network (R20 and R21) to again with a 4.7µF capacitor and 330kΩ resistor suggests bring it into the negative-voltage range needed for in- that this will roll off signals below about 0.10Hz. Re- put to one of these common-emitter stages, and then ally, the input impedance of whatever it’s plugged Q6, R22, and R23 amplify and invert it again. The into, appearing in parallel with R24, will have the Q6 amplifier stage has a little bit less voltage gain effect of reducing the resistance and increasing the because of the smaller resistor R22, but otherwise is corner frequency; but even driving a low-impedance identical in operation to the Q5 stage. And the result “passive” module it should pass all reasonable audio is used directly as the module’s DC-coupled output. frequencies. The real point of R24 is not so much to The main reason for using two amplifier stages create a controlled roll-off frequency as to drain away here is so that the module’s overall response will be any static charges that might from time to time ap- non-inverting. Another reason is that, despite the pear on the output pin; and the point of the network loss in the coupling network between the two stages, as a whole is to block DC. doing it this way allows each single amplifier to work

20 Mechanical drawings On the following pages you will ﬁnd: • the schematic diagram for the module; • a mock-up of what the completed module looks like from the front panel; • the top-side silk screen art showing component placement; • the bottom-side silk screen art showing component placement (note this drawing is mirrored, and shows what you actually see looking at the board, not the X-ray view used in other Kicad output); • a mechanical drawing of the front panel showing the locations and sizes of the holes in it; and • an exploded isometric drawing showing how the boards and hardware ﬁt together.

21 1 2 3 4 5 6 7 8

+12V

A R6 100k A +12V 1 D1 1N5818 J1 2 3 input 1 Q1 PWR_FLAG positive offset norm 3 100k 2 2N5088 J7 + R2 C2 C4 9 10 R12 47k 10uF 0.1uF

1 7 8 5 6 PWR_FLAG 3 4 1 2 R8 C3 + C5 22k 10uF 0.1uF

PWR_FLAG D2 1N5818

-12V -12V +12V

B B 1

2 3 input 2 J2 Q2 0 norm 3 100k 2 2N5088 R3 R13 47k 1

R9 22k

+12V

+12V -12V

+12V R22 4.7k C1 4.7uF R18 1 5.9k 1

C 3 C input 3 J3 2 2 J6 0 norm Q3 R20 22k Q6 3 100k 2 2N5088 2N5088 R24 3 R4 1 330k R14 47k Q5 2 J5

1 2N5088 3 AC output R21 12k R17 R10 R23 2.7k 22k 510 DC output

R19 510 -12V

-12V -12V -12V +12V -12V

2 3 J4 Q4 D D input 4 3 100k 2 2N5088 negative offset norm R5 R15 47k 1

R1 100k R11 22k

-12V H1 H3 M3x6 nylon -12V H2 H4 M3x6 nylon

+12V

E 1 E R16 330k R7 2 100k 3

-12V

MSK 011 Transistor Mixer $Id: discrmix.sch 8239 2020-10-07 21:02:23Z mskala $ 1/1 F F 1 2 3 4 5 6 7 8 MSK 011 transistor mixer

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